1. Field of the Invention
The present invention relates to apparatus for determining the liquid level in a non-metallic tank and, more particularly, to externally mounted means for determining and providing a remote indication of the liquid level in a tank, such as a waste water tank for a recreational vehicle.
2. Description of the Prior Art
A wide variety of different types of liquid containing tanks are provided with measurement means for determining the liquid level therein. Usually, such measurement means are positioned within the tank and directly contact the surface of the liquid in order to determine the level thereof. On the other hand, in certain cases, such as in the measurement of the liquid level in waste water holding tanks used in recreational vehicles and the like, it is desirable that the measurement means not require an object to contact the liquid inside the tank or require holes to be drilled in the tank. Any obstruction or metallic object inside of a waste water tank will be subject to strong corrosive action and will become entangled with solid or semi-solid materials, causing a malfunction of the measuring device and/or preventing the tank from being completely emptied. This also leads to sanitary problems and unpleasant odors in the case of waste water holding tanks.
In order to determine the liquid level in a tank without contacting the liquid therein, it has been proposed to mount metal electrodes, probes, or plates on the tank, which plates are insulated from the liquid therein in a manner such that the capacitance between the plates varies as a function of the level of the liquid in the tank. This change in capacitance can then be utilized to provide an indication of liquid level. However, problems have been encountered with this type of system which has rendered it impractical for use heretofore.
For example, the liquid in a waste water holding tank typically has a fairly high viscosity and is very conductive because of the presence of human wastes and other solid or semi-solid materials. As a result, the tank contents tend to adhere to the walls of the tank. Whenever the tank is agitated or filled and then emptied, the walls become coated with a film of this highly conductive substance. As a result, the amount of signal current which can pass between two electrodes, probes, or plates is increased over the amount which would flow solely due to the liquid level in the tank, causing an incorrectly high reading. This situation can prevail for hours or even days until the inside surfaces of the tank walls have dried.
Still further, the various techniques that have been used heretofore to mount electrodes, probes, or plates on or in tanks and the configurations of such elements have not been conducive to the production of accurate and linear output readings.